Fuels and chemicals produced from synthesis gas (syngas) or CO-containing industrial off-gas represent a prime alternative to fossil fuel and chemicals derived thereof. Chemical catalytic conversion of these gases into fuels or chemicals is expensive or commercially unattractive. Instead, biological conversion of these gases into fuels and chemicals (known as gas fermentation), have several advantages over catalytic processes, including higher specificity, higher yields, lower energy costs and greater resistance to poisoning.
The efficiency of gas fermentation is known to be limited primarily by a low gas-liquid mass transfer rate due to the poor solubility of gaseous substrates (for example, CO and H2) in liquids. The mass transfer efficiency, or volumetric mass transfer rate, is provided as follows:
      -                  ⅆ        N                              V          R                ·                  ⅆ          t                      =            k      L        ⁢          a      ⁡              (                              C            *                    -                      C            L                          )            
Where
  -            ⅆ      N              ⅆ      t      is the rate at which the gaseous substrate is transferred to the liquid phase; KLa is the volumetric mass transfer coefficient, consists of the liquid side mass transfer coefficient kL and the specific mass transfer surface area, a. C* is the saturation concentration of the gas in the liquid (i.e., the solubility) which is proportional to the partial pressure of the gaseous substrate and CL is the actual gas concentration in the liquid, the difference between the two, i.e., (C*−CL) is the mass transfer driving force. Under pure mass-transfer limited conditions, CL≈0. VR is the wetted volume of the reactor and it is the sum of gas volume and liquid volume.
Thus, in order to improve mass transfer efficiency, one needs to either increase kLa or the driving force. The driving force can be enhanced by using higher pressure; however, such methods are of high cost as the compression of gas is required. It is generally more preferable to increase kL and/or a. While kL is an intrinsic property of the liquid and gas, meaning it is difficult to change, a has a simple relationship with the gas holdup, εG, and the average bubble radius, rb, both of which can be easily manipulated. The relationship is as follows:
  a  =            3      ⁢              ɛ        G                    r      b      
The above equation dictates that the specific mass transfer area can be increased by an increase in gas holdup, εG, or a decrease in bubble size, rb, or a combination of both. Unfortunately, most of such methods tend to generate a large quantity of foam, which may block the pipelines downstream of the bioreactor. Thus, when measures are taken to increase the mass transfer surface area, special attention must to be paid to foam control.
A high mass transfer rate is generally desirable for gas fermentation. However, the process can suffer from substrate inhibition if the mass transfer rate is higher than the maximum reaction rate the microbes can provide. For example, a high dissolved CO concentration results in slow growth of microbes and slow uptake of H2, and if such conditions last for a prolonged period of time, the culture may slowly die out (Design of Bioreactors for Coal Synthesis Gas Fermentations, J. L. Vega, E. C., Clausen and J. L. Gaddy, 1990, Resources, Conservation and Recycling, Vol 3, Pages 149-160; Effect of CO partial pressure on cell-recycled continuous CO fermentation by Eubacterium limosum KIST612, I. S. Chang, B. H. Kim, R. W. Lovitt, J. S. Bang, 2001, Process Biochemistry, Vol 37, Page 411-421). Such “oversupply” conditions may occur globally in a small scale, well-mixed reactor, but may also occur locally in a large scale reactor where there is high local dissolved CO concentration, typically at the bottom where the gas is introduced and the CO partial pressure is high.
Therefore, a commercial scaled reactor for gas fermentation needs to provide a high gas-to-liquid mass transfer rate, and also needs to be flexible in order that the mass transfer rate can be regulated when necessary. Effective foam control is also a requirement.
At bench-top scale, gas fermentation is typically carried out in continuous stirred tank reactors (CSTR). However, these are inappropriate for commercial scale application due to high energy consumption and other concerns. Instead, bubble columns with or without internal or external loops may be used for large scale gas fermentation. Forced-circulation external-loop reactors are a type of bubble column reactor where the liquid is forced to circulate between a main column (the riser) and an external loop (the downcomer) by a pump, herein referred to as a loop pump.
In known forced-circulation loop-reactor configurations, the speed of the loop pump has two major effects on the hydrodynamics and mass transfer of the system: (a) an increase in loop pump speed enhances the gas entrainment from the riser to the downcomer, which tends to increase the riser and downcomer holdup, and thus improves mass transfer; (b) an increase in loop pump speed increases the liquid velocity in the riser, which tends to wash out the gas bubbles in the riser quickly and decreases the gas holdup and reduces the gas residence time. Conversely, if the loop pump speed is reduced, the gas bubbles in the riser can stay for a longer period of time, but the gas entrainment into the downcomer will be substantially less, which could reduce the reaction rate in the downcomer and the overall performance of the reactor. In addition, as the gas introduced at the bottom of the riser has high CO content, a low loop pump speed in a deep reactor aggravates substrate inhibition.
Thus, a loop pump is ineffective in terms of regulating the mass transfer due to its competing effects on gas entrainment and riser liquid velocity. It is an object of the present invention to provide a means of decoupling the two competing effects of the loop pump and to provide more effective mass transfer regulation therein, as well as enhanced foam control and lower overall energy consumption. Furthermore, the present invention overcomes disadvantages known in the art and provides the public with new methods for the optimal production of a variety of useful products. Even minor improvements to a gas fermentation process or system for producing one or more products can have a significant impact on the efficiency, and more particularly, the commercial viability, of such a process or system.